(a) Field of the Invention
The present invention relates to a plasma display panel (PDP) video processing circuit, a video display device using the video processing circuit, a video processing method, and a video display method using the video processing method.
(b) Description of the Related Art
Image qualities of PDPs such as brightness and contrast have been improved, and they have come to be used for large-screen flat displays.
AC PDPs use a subfield method for controlling lighting of pixels in order to represent gray scales. FIG. 5 shows a subfield sequence for displaying 256 gray scales with eight subfields. As shown, each subfield has an address period and a sustain period. All of the subfields have the same address period but different sustain periods. Numbers provided below the respective subfields are weights of the corresponding subfields The number of sustain pulses assigned to the subfields is increased by the ratio of weights, and the sustain periods are lengthened according to the increase in the number of sustain pulses. PDPs typically display gray scales by combining the lighting and non-lighting of the respective subfields.
FIG. 6 shows combinations of gray scales that are actually displayed when representing the 256 gray scales and the lighted subfields. The subfields indicated with a “?” are the lighted subfields.
An address period is inserted between the sustain periods of each subfield. This discontinuous lighting appears as if the lighting of each subfield is integrated and continuous because of an “afterimage effect” to human eyes. When the total amount of lighting is varied because subfields are combined with different weights, it is deemed to be a variation of brightness, which represents the gray scales.
However, this gray scale representation method causes a “contour noise” phenomenon, which occurs when displaying video with the subfield method. Contour noise occurs where a lighting scheme is greatly varied. In this instance, the lighting scheme represents a combination of different subfields.
A conventional method for detecting motion between frames and applying the gray scales with consecutive lighting schemes to the detected motion has been proposed in order to solve the contour noise problem by Kawahara Isao and Sekimoto Kunio, “Developments of suppressing dynamic false contour for fine PDP” in the annual image information media transactions, pp. 369-370, published in 2000. This method is effective in that the contour noise is reduced while maintaining the current display performance.
FIG. 7 shows an example of a screen on which the gray scales are represented by using a subfield sequence. As shown, a square window of the 128/255 gray scale is displayed on the background of the gray scale of 127/255. When the images on the screen are still, normal gray is represented by the combination of the subfields with different weights.
FIG. 8 shows the window of the gray scale of 128/255 of FIG. 7 scrolled to the right on the screen. A dark portion and a bright portion which are not shown when the window is not moved, appear at the front and the end of the window.
The dark portion and the bright portion are referred to as the contour noise. The unintended bright portion and the dark portion are displayed with a gray difference of 1 with gradually consecutive variation of the gray scales in the image. As such, this phenomenon substantially damages the quality of the video display.
Widths of the bright portion and the dark portion vary according to the speed of which the window moves, and the widths are generally widened as the window moves faster.
FIG. 9 illustrates how contour noise is generated. The subfield numbers SF1 to SF8 represent lighted subfields and correspond to the subfield number of FIGS. 5 and 6. The whole one-field period is not occupied by the sustain period in a precise temporal manner, that is, a light emission time since the address period is actually provided, but for ease of description, the one-field period is illustrated in FIG. 9 to be occupied by the light emission time.
Human eyes follow the front portion (depicted by an arrow) of the moving window with the gray scale of 128/255, but a person who looks at the front portion initially sees the lighting of gray scale of 127 on the background because of the afterimage effect, and then sees the lighting of the window with the gray scale of 128, as shown in FIG. 9.
As can be seen from FIG. 9, the combination of the lighted subfields is substantially modified in the case of the gray scales of 127 and 128, and hence, the time without light emission is increased at the variation point of the gray scale from 127 to 128 as shown in FIG. 9. Accordingly, the brightness of the front portion is reduced, and the dark portion shown in FIG. 8 appears.
An opposite phenomenon occurs at the rear portion of the window, and the bright portion of FIG. 8 appears since the light emission of the gray scales of 127 and 128 approach.
Generation frequencies of the contour noise are somewhat predictable. The generation of contour noise increases at the boundary of the gray scales with greatly varied light emission schemes, such as between the gray scales 7 and 8, and between the gray scales 15 and 16 from the subfield arrangement tables shown in FIGS. 6 and 10.
Accordingly, it is possible to effectively reduce the contour noise near the gray scales at which the contour noise is generated by detecting and processing the motion of the gray scales.
As shown in FIG. 11, a method for reducing the contour noise is to display the moving pixels by only using the gray scales which have continuous light emission schemes and have no unlighted subfields between the gray scales. This method is effective for reducing the contour noise because it uses lesser variations of the light emission schemes.
The case of displaying the gray scale of 256 from the 8 subfields is partially illustrated in FIG. 11. The gray scales with the consecutive light emission schemes include nine gray scales of 0, 1, 3, 7, 15, 31, 63, 127, and 255. Accordingly, a multiple gray scale processing method such as error diffusion is used in order to represent 256 gray scales. However, this method generates rough gray scale representations when the gray scales are displayed using only restricted gray scales for all the pixels in the multiple gray scale processing method.
Therefore, a field memory is used as shown in FIG. 12 to compare the current-field signal with the immediately previous field signal to determine whether motion is indicated by the difference of their magnitude. When no motion is indicated, the input video signals are output as they are. When motion is indicated, the signals which are processed by using the consecutive gray scales are output.
This conventional contour noise reduction method adds some efficiency to the image display, but it may generate an opposite effect, which will now be described.
FIG. 10 shows combined contents of gray scales and lighted subfields of FIG. 6 with reference to the actual light emission time. FIGS. 13(a), 13(b), and 13(c) show the combination of the gray scales 7 and 8 in the window of FIGS. 7 and 8. FIG. 13(a) shows a previous field, FIG. 13(b) shows a current field, dotted lines provided over FIGS. 13(a) to 13(c) show a shift of the window between two fields, and slanted areas indicated as A and B in FIG. 13(c) show areas which are determined to have motion according to motion detection results.
The results determined to “have motion” are reflected on the current field, and gray scales with consecutive light emission schemes are applied to the background's corresponding positions near the right and left portions in the window of FIG. 13(b) corresponding to the areas A and B.
FIG. 14 shows gray scales of previous fields of the positions of the areas A and B of FIG. 13(c) and the gray scales applied to the current field. As to the area A, the gray scale 8 is represented by using the gray scales 7 and 15 and performing an error diffusion process in order to represent the gray scale 8 with only gray scales having consecutive schemes. As to the area B, the gray scale 7 is represented as it is since the gray scale 7 has consecutive schemes.
The combinations of the gray scale of the previous fields and the gray scale of the current fields of the area A are compared referring to FIG. 10. Since the gray scale 7 of the previous field corresponds to the gray scales 7 and 15 in the current field, all the light emission schemes in FIG. 10 are consecutive, and hence, the light emission schemes are less scattered and the contour noise is reduced compared to the gray scale 8 of the original current field.
However, regarding the area B, the contour noise is not reduced since the previous field has the gray scale 8 having a light emission scheme with a gap even if the current field is represented with gray scales having consecutive light emission schemes. Because the previous field has already been displayed, it cannot be processed like area A to have gray scales with consecutive schemes.